A database script was also needed to modify that property in the items already affected by this bug.
The following worked for us; since our teachers always input peer review assignment instructions when they make use of a peer review assignment

UPDATE assignment_assignment
SET xml = REPLACE(xml, 'allowpeerassessment="false"', 'allowpeerassessment="true"') WHERE xml like '%allowpeerassessment="false"%'
and xml not like '%peerassessmentinstructions=""%';

Otherwise, the last part

and xml not like '%peerassessmentinstructions=""%'

should be modified to make sure that we are making peer review assignments only the ones that actually are.

Miguel Pellicer, at the moment we discovered the bug, Master was unstable regarding assignements and a lot of issues were being tackled in a sprint.
It wasn't possible to test this because it was not even possible to duplicate assignments: (for example see SAK-33318).
So we decided to fix it for our 11.4 and wait for the master branch to be stable again.